RESPONSE TO SMALLPOX VACCINE IN PERSONS IMMUNIZED IN THE DISTANT PAST

Abstract

Frey SE, Newman FK, Yan L, Lottenbach KR, Belshe RB. JAMA. 2003;289:3295–3299There is renewed interest in the use of smallpox vaccine, because of the potential for a bioterrorist attack. One obvious question relates to the current status of people who have been vaccinated in the past. This study sought to evaluate the use of diluted vaccinia virus for vaccination of previously vaccinated (nonnaive) participants.Eighty nonnaive participants, 32 to 60 years of age, were randomized to receive either undiluted or diluted (1:3.2, 1:10, or 1:32) doses of smallpox vaccine, in a single-blinded study. A comparison group of 10 vaccinia-naive participants, 18 to 31 years of age, received undiluted vaccine.Participants were enrolled between April 1 and May 15, 2002, at the National Institute of Allergy and Infectious Diseases Vaccine and Treatment Evaluation Unit at St Louis University (St Louis, MO). Smallpox vaccine was administered through scarification, using 15 skin punctures in the deltoid region of the arm. Outcome measures included the presence of a major reaction, defined as a vesicular or pustular lesion or area of palpable induration surrounding a central lesion, after vaccination, measures of viral shedding, and antibody titers.Initial vaccination resulted in a major reaction for 64 of 80 nonnaive participants. Ninety-five percent of nonnaive participants had major reactions in the undiluted group, 90% in the 1:3.2 dilution group, 81% in the 1:10 dilution group, and 52.6% in the 1:32 dilution group. All of the vaccinia-naive participants (n = 10) experienced major reactions. Compared with vaccinia-naive participants, nonnaive participants had significantly smaller skin lesions (P = .04) and a significantly lower incidence of fever (P = .02). Preexisting antibody was present in 76 of 80 nonnaive participants. Antibody responses were significantly greater and occurred more rapidly among the nonnaive participants, compared with the vaccinia-naive participants (P = .002 for day 28 and P = .003 for 6 months). Vaccinia-naive participants shed virus from the vaccination site 2 to 6 days longer and had significantly higher mean peak viral titers, compared with the nonnaive participants (P = .002).Previously vaccinated persons could be successfully revaccinated with diluted (≤1:10) smallpox vaccine. Fewer adverse reactions were observed in this study among nonnaive participants, compared with vaccinia-naive participants, which might be attributable to immunologic memory.This is an excellent practical study addressing an important issue. Although we hope that widespread smallpox vaccination will not be necessary, these data should be useful for the design of whatever program might be necessary.